Liquid crystal displays (LCDs) are typically comprised of two flat glass substrates that encapsulate a thin layer of liquid crystal material. Arrays of transparent thin-film electrodes on one of the substrates modulate the light transmission properties of the liquid crystal material, thereby creating the image. By incorporating an active device such as a diode or thin film transistor (TFT) at each pixel, high contrast and response speed can be achieved to produce high-resolution displays. Such flat panel displays, commonly referred to as active matrix LCDs (AMLCD), have become the predominant technology for high performance displays such as computers and televisions.
The fabrication process for LCDs, and especially those used in the manufacture of poly-crystalline silicon (poly-Si) displays, typically consists of successive deposition and patterning of thin films using elevated temperature processes which result in substrate heating. Because of the high registration requirement between patterning steps for these thin films, the glass substrates often require dimensional stability (low shrinkage) in the 5-20 parts per million (ppm) range throughout the process. Five to twenty parts per million shrinkage means, for example, 2.5-10 microns shrinkage over a substrate length of 500 mm. When greater than 5-20 ppm shrinkage occurs, registration errors will accrue between components subsequently applied.
Poly-Si is conventionally made by depositing amorphous silicon (a-Si) onto a glass substrate using chemical vapor deposition (CVD) techniques, and subsequently exposing the coated glass to high temperatures for a sufficient period of time to crystallize the a-Si to poly-Si. This crystallization step is typically done at about 600° C. for several tens of hours. In addition, several other high temperature processes may follow the crystallization step. Such process steps include deposition and annealing of the gate oxide, and source/drain annealing.
The relatively high temperatures of the crystallization and subsequent processing steps encountered during poly-Si TFT manufacturing greatly increases the potential for glass substrate shrinkage.
Manufacturers of glass substrates (e.g., liquid crystal display, or “LCD”, glass substrates) often heat treat the glass substrates to pre-shrink or compact the glass prior to shipping. Compacting glass substrates can be performed at various temperatures below the glass substrate strain point. Compaction or densification is performed to minimize dimensional changes of the glass during the customer's processing of the glass substrate. If the glass substrates are not pre-shrunk, the substrates can undergo contour changes that may negatively affect the finished display quality. Compaction must be performed without creating glass chips that can contaminate the glass surfaces, or distorting the glass substrate surfaces through spatially non-uniform heating and/or cooling patterns.
Conventionally, a closed cassette has been used to support glass substrates during heat treatment. An open cassette is also utilized in some applications. In a closed cassette support method, multiple glass substrates are held in a vertical orientation within enclosed sections of a cassette. The glass substrates are supported with horizontal and vertical supports (such as those made of stainless steel). In practice, the glass substrates are supported around their perimeters to maintain surface quality and prevent warp. The glass substrates are typically captured along the full length of all four edges.
In an open cassette support method, multiple glass sheets are held in a vertical orientation within a cassette. The glass sheet is supported at its edges with vertical and horizontal supports. As in the closed cassette support method, the glass substrate is supported around the perimeter to maintain its physical attributes. Both the open and closed cassette methods generally minimize the gravity effect on the glass during heat treatment.
In both the closed and open cassette support designs, the glass substrates are contacted along substantially all of at least three edges. This contact often causes substrate damage or loss. The full-contact supports also have an impact on the thermal characteristics of the system. As may be appreciated, the metal mass of the supports concentrated along each substrate edge impacts the temperature profile at the edges due to the heat having to travel through metal before reaching the glass along the edges and corners. Additionally, in both support designs, debris (including glass particles and chips) builds up in the bottom-edge support and is difficult to clean out; as a result, these support designs can cause significant debris contamination of glass substrates. Moreover, the large differences in coefficient of thermal expansion between the metal supports and the glass substrates result in a large movement of the substrates relative to the supports and potential damage to the substrates.
Both of the aforementioned support designs are manufactured by bending and forming sheet material (such as stainless steel) into the required assembly. By nature, these procedures are not precise, difficult to produce, and costly to manufacture.